Low-frequency magnetic screening made from a soft magnetic alloy

ABSTRACT

The invention relates to a magnetic screening for frequency fields between 50 Hz and 3000 Hz, made from a soft magnetic alloy of the following composition in wt. %: 30%≦Ni≦40%, 0%≦Cu+Co≦4%, 5%≦Cr+Mo≦17%, 5%≦Cr 0%≦Nb≦2%, Mn≦0.35%, Si≦0.2%, C≦0.050%, O≦0.0160%, S≦0.0020%, B≦0.0010%, optionally at least one element selected from magnesium and calcium in amounts such that the sum thereof remains below 0.1 %, the rest being iron and production impurities. The chemical composition furthermore satisfies the following relationships: Cr+Mo≦0.8×Ni+0.9×(Co+Cu) 18.4; Cr+Mo≦4×Ni+3×(Co+Cu)−124; 4×(Cr+Mo)≧125−3×Ni. The invention further relates to use of said alloy for the production of low-frequency magnetic screening.

The present invention relates to low-frequency magnetic shielding made from a soft magnetic alloy and to the use of this alloy for the production of low-frequency shielding. In the context of the present invention, the term “low-frequency” denotes frequencies between 50 Hz and 3000 Hz.

Magnetic shielding is produced from a high-permeability magnetic alloy and, in particular, from an alloy of the Fe—Ni80 type containing approximately 80% of nickel. They have permeability in a direct field μ_(cc) of more than 100000 and permeability in an alternating field at 300 Hz, μ_(300 Hz) of more than 10000. Furthermore, these alloys have a coercive field Hc of less than 20 mOe and saturation induction Bs of more than 6000 Gauss. However, these alloys are very expensive as they have a high nickel content.

Alloys of the Fe—Ni36 type containing approximately 36% of nickel are used to produce less expensive shielding. However, these alloys have permeability in a direct field μ_(cc) between only 20000 and 30000 and permeability in an alternating field at 300 Hz, μ_(300 Hz) between 8000 and 9000, a coercive field Hc between 50 and 100 mOe and saturation induction Bs of approximately 13000 Gauss. With these magnetic properties the shielding obtained is less effective than shielding produced from Fe—Ni80 alloy.

It has also been proposed, for example in U.S. Pat. No. 5,158,624, to use an alloy containing 35 to 40% of nickel and 5 to 14% of chromium, the remainder being iron and impurities and the composition satisfying the relationships 3×Ni−5×Cr≦80 and Ni−Cr≧25. Furthermore, the contents of oxygen, sulphur and boron have to be strictly controlled; in particular the oxygen content has to be kept at less than 0.005%. In addition, the alloy contains 0.5% manganese, approximately 0.2% of silicon, approximately 0.01% of aluminum. This alloy has permeability in an alternating field at 300 Hz, μ_(300 Hz) of between 9400 and 14900, a coercive field Hc between 10 and 80 mOe and saturation induction Bs between 5000 G and 8200 G.

This alloy has the advantage of having higher permeability μ_(300 Hz) than the alloy Fe—Ni36 and of containing chromium, and this gives it some resistance to corrosion, but its better permeability may only be obtained with very low oxygen contents, and this restricts the production thereof. In addition, it would be desirable to have an inexpensive alloy having even better magnetic permeability.

The object of the present invention is to propose an inexpensive soft magnetic alloy, which is suitable for the production of low-frequency magnetic shielding, is more effective and less restrictive to produce than known alloys.

The invention accordingly relates firstly to magnetic shielding for frequency fields between 50 Hz and 3000 Hz, made from a soft magnetic alloy of the following composition in % by weight: 30%≦Ni≦40% 0%≦Cu+Co≦4% 5%≦Cr+Mo≦17% 5%≦Cr 0% ≦Nb≦2% Mn≦0.35% Si≦0.2% C≦0.050% O≦0.0160% S≦0.0020% B≦0.0010%, optionally at least one element selected from magnesium and calcium in amounts such that the sum thereof remains below 0.1%, the remainder being iron and production impurities, the chemical composition furthermore satisfying the following relationship: Cr+Mo≦0.8×Ni+0.9×(Co+Cu)−18.4 Cr+Mo≦4×Ni+3×(Co+Cu)−124 4×(Cr+Mo)≧125−3×Ni.

Preferably, it is preferable for the silicon content to be less than 0.15%, for the manganese content to be more than 0.05%, and for the sum of cobalt and copper contents to be more than 0.015%. The oxygen content may be more than 0.0050%.

The invention will now be described in more detail and illustrated by examples.

The alloy according to the invention contains the following, in % by weight:

-   -   more than 30% of nickel for obtaining good magnetic properties         and, in particular, adequate magnetic permeability and         saturation induction, but less than 40%, because nickel is an         expensive element and, above 40%, does not improve the desired         magnetic properties;     -   one or more elements selected from copper and cobalt, the sum of         their contents being between 0% and 4% and preferably more than         0.015%, and more preferably more than 0.5%, and yet more         preferably more than 1% to increase the saturation induction Bs         and obtain high magnetic permeability when the nickel content is         relatively low;     -   one or more elements selected from chromium and molybdenum, the         sum of their contents being between 5% and 17%, and the chromium         content being more than 5%. These elements increase the magnetic         permeability and reduce the coercive field, providing the         contents thereof are not too high. Furthermore, to obtain the         desired magnetic properties, namely Bs>4000 G at 40° C. and good         magnetic permeability, the Cr, Mo, Ni, Cu and Co contents have         to be such that:         Cr+Mo≦0.8×Ni+0.9×(Co+Cu)−18.4         and Cr+Mo≦4×Ni+3×(Co+Cu)−124         and, to have good magnetic permeability, the Cr, Mo and Ni         contents have to be such that:         4×(Cr+Mo)≧125−3×Ni;     -   optionally up to 2% of niobium to increase the mechanical         strength;     -   less than 0.35% and preferably more than 0.05% of manganese and         less than 0.20% and preferably less than 0.15% of silicon. These         elements are required for production, but the inventors have         found, in a novel manner, that, by limiting the contents of         these elements, the magnetic permeability at 300 Hz, μ_(300 Hz),         is substantially increased, even with oxygen contents which may         be as high as 0.0160%;     -   less than 0.0500% of carbon, less than 0.0020% of sulphur, less         than 0.0010% of boron, less than 0.0200% of nitrogen and less         than 0.0160% of oxygen. These limits to the contents of         impurities enable high magnetic permeability to be obtained. It         should be noted, however, that the oxygen content may be more         than 0.0050%, without adversely affecting the magnetic         properties, and this enables the alloy to be produced more         easily and more economically, which is desirable;     -   optionally magnesium or calcium in amounts of which the sum may         be as high as 0.1000% and must preferably be less than 0.0500%,         but more than 0.0010%, in order to form magnesium or calcium         oxides which facilitate the mechanical cutting of parts in         strips.

The remainder of the composition is iron and optionally impurities.

Using this alloy, strips are produced, for example, by hot rolling then cold rolling. At the final thickness, the strips are subjected to annealing at least at 1050° C. and preferably at more than 1100° C., also preferably in a hydrogen reducing atmosphere or in a mixture of steam and hydrogen. After annealing, cooling to ambient temperature preferably has to be carried out at slow speed, in other words necessitates more than 1 hour to be able to reach 200° C. in order to optimize the magnetic permeability at 300 Hz.

The following magnetic properties are obtained in the strips obtained in this way and also having a thickness of 0.4 mm: μ_(300 Hz)>15000 μ_(cc)>40000 Bs>4000 G Hc<100 mOe

These properties allow production of magnetic shielding which is very effective in low-frequency fields, but also in direct fields (for example, terrestrial field).

As an example, the alloys designated 1 to 21 according to the invention were produced and the alloys designated 22 to 32 were provided as a comparison. The compositions and the properties of these alloys are shown in Tables 1 and 2, and the magnetic properties of the alloys are shown in Tables 3 and 4.

The magnetic properties were measured on 0.6 mm thick strips in the case the coercive fields Hc, expressed in mOe, and in the case of permeability in a direct field gcc which was measured at 0° C. and at 40° C. The saturation induction Bs, expressed in Gauss, was measured at 40° C. The magnetic permeability in an alternating field at 30 Hz, μ_(300 Hz), was measured at 40° C. on 0.4 mm thick strips. The alloys were produced under vacuum in an induction furnace then cast in the form of hot-rolled then cold-rolled ingots to provide strips from which samples were cut and were then annealed for four hours at 1170° C. under pure dry hydrogen, with rapid cooling if they were intended to measure permeability in a direct field and slow cooling if they were intended to measure permeability in an alternating field.

Alloys 1 to 21 all have a coercive field of substantially less than 100 mOe, permeability in a direct field of more than 40000, at both 0° C. and 40° C., permeability in an alternating field at 300 Hz of more than 15000 and saturation induction of more than 4000 G. TABLE 1 In % by weight In ppm Item Ni Cr Mo Co Cu Mn Si Nb C S P N O B 1 38.94 9.14 <0.005 0.042 0.022 0.323 0.163 traces 65 13 41 28 67 <10 2 36.52 9.02 0.0057 0.065 0.020 0.315 0.168 traces 55 14 43 27 80 <10 3 36.88 9.04 <0.005 0.044 0.022 0.310 0.155 traces 45 14 43 27 100 <10 4 37.64 6.98 <0.005 0.005 0.017 0.333 0.144 traces 45 14 <30 13 93 <10 5 33.66 7.95 — — — 0.188 <0.01 — 41 9 34 29 120 <10 6 33.55 8.17 <0.005 0.014 <0.01 0.172 0.016 traces 160 <5 <30 21 35 <10 7 37.63 9.31 0.023 0.503 0.094 0.293 <0.01 0.007 86 8 <30 27 90 <10 8 37.95 9.56 0.0056 1.42 0.020 0.289 0.017 traces 83 9 <30 30 84 <10 9 37.86 10.55 <0.005 0.962 0.018 0.299 0.019 traces 49 10 <30 27 140 <10 10 39.49 9.6 0.0097 1.02 <0.01 0.287 0.021 0.006 96 10 30 29 29 <10 11 37.75 9.54 — 1.02 — 0.300 — — 91 6 <20 6.6 62 <10 12 37.66 9.19 <0.005 1.02 <0.01 0.178 0.105 traces 150 6 <30 14 25 <10 13 35.8 9.05 — 1.04 — 0.300 — — 83 <5 <20 <5 99 <10 14 35.7 9.17 <0.005 1.03 <0.01 0.173 0.111 traces 130 <5 <30 12 64 <10 15 35.77 5.6 <0.005 1.01 <0.01 0.306 0.035 traces 94 <5 40 8.3 75 <10 16 37.74 5.76 <0.005 0.969 <0.01 0.308 0.033 traces 92 <5 38 7.5 58 <10 17 35.85 5.89 <0.005 2.85 <0.01 0.308 0.031 traces 83 <5 34 5.5 52 <10 18 35.79 8.92 — 3.03 — 0.290 — — 90 <5 <20 9.6 69 <10 19 37.77 5.8 0.0086 2.87 <0.01 0.298 0.033 traces 69 <5 37 8.8 83 <10 20 37.45 8.72 — 3.06 — 0.300 — — 89 <5 <20 12 68 <10 21 31.84 8.23 <0.005 3.07 <0.01 0.174 0.013 traces 150 6 <30 10 19 <10

Alloys 22 to 32, given as a comparison, show the significance of the limits imposed on the chemical composition. TABLE 2 By % weight In ppm Item Ni Cr Mo Co Cu Mn Si Nb C S P N O B 22 31.84 8.2 <0.005 0.011 <0.01 0.173 0.018 traces 150 5 <30 10 24 <10 23 33.46 4.88 0.014 0.014 0.011 0.133 0.018 traces 66 11 <30 19 94 <10 24 33.78 2.02 2.03 traces <0.01 0.186 <0.01 traces 150 <5 <30 10 13 <10 25 33.78 0.019 2.21 traces <0.01 0.183 <0.01 traces 130 <5 <30 10 34 <10 26 37.69 3.14 <0.005 1.06 <0.01 0.296 0.031 traces 90 <5 35 <5 57 <10 27 33.7 8 — 2.07 — 0.187 <0.01 traces 66 15 38 7 180 <10 28 33.96 2.64 <0.005 1.96 <0.01 0.259 0.032 — 89 <5 35 5.1 85 <10 29 33.83 5.1 <0.005 2.02 <0.01 0.152 <0.01 traces 67 7 <30 7.2 110 <10 30 31.68 8.03 0.027 0.01 2.97 0.176 0.018 traces 120 7 31 54 67 <10 31 30.14 2.09 <0.005 traces 2.99 0.193 <0.01 0.005 130 <5 <30 10 29 <10 32 32.29 1.87 0.082 traces 3.92 0.166 0.014 0.007 83 9 35 7.6 86 <10

Alloy 22 has a chromium content which is too high to satisfy the conditions Cr+Mo≦0.8×Ni+0.9×(Co+Cu)−18.4 and Cr+Mo≦4×Ni+3×(Co+Cu)−124, and its saturation induction is very low.

Alloys 23, 24, and 25 have chromium contents which are too high to satisfy the condition 4×(Cr+Mo)≧125−3×Ni, and their permeability in a direct field is substantially less than 40000.

Alloy 26 does not satisfy the relationship Cr≧5%, and its permeability in an alternating field at 300 Hz is substantially less than 15000.

Alloy 27 has an oxygen content of more than 160 ppm and its permeability in a direct field is substantilly less than 40000.

Alloys 28 and 29 do not satisfy the relationship 4×(Cr+Mo)≧125−3×Ni, and alloy 28 does not satisfy the condition Cr≧5%. On the one hand, their permeability in a direct field is substantially less than 40000, but, in particular, their permeability in an alternating field is substantially less than 15000.

Alloy 30 does not satisfy the conditions Cr+Mo≦0.8×Ni+0.9×(Co+Cu)−18.4 and its permeability in a direct field is substantially less than 40000.

Alloy 31 does not satisfy the conditions Cr+Mo≦4×Ni+3×(Co+Cu)−124 and 4×(Cr+Mo)≧125−3×Ni, and its magnetic permeability is inadequate in both an alternating field and a direct field. TABLE 3 Magnetic properties Item Hc Bs μ_(cc) 0° C. μ_(cc) 40° C. μ_(300 Hz) 1 28 6800 61400 62400 17900 2 23 5500 64000 50300 23200 3 22 6000 69000 59600 21700 4 39 8100 69200 67200 21100 5 18 4500 47500 44700 17800 6 10 4200 80300 72000 16000 7 22 6500 67600 67400 21400 8 26 6700 66800 64700 19100 9 22 5600 48100 77200 20800 10 21 7200 72600 72100 18600 11 22 6800 98300 93000 18500 12 14 6800 132900 118300 22000 13 24 5700 71500 66900 22100 14 22 5700 86900 78900 22400 15 44 8400 42700 57100 17900 16 34 9500 74900 93400 19400 17 45 9100 52500 60500 17000 18 27 6700 84200 97600 19100 19 44 10000 48100 68200 17600 20 25 7500 91900 84100 20800 21 13 4700 51600 65000 17900

TABLE 4 Magnetic properties Item Hc Bs μ_(cc) 0° C. μ_(cc) 40° C. μ_(300 Hz) 22 7 500 45800 — 13600 23 36 7000 24200 29100 14400 24 28 7400 33300 31900 14200 25 33 8200 29100 26600 14100 26 54 11700 48200 59200 13700 27 32 5900 26600 37600 20000 28 85 10200 18000 22900 10800 29 69 8400 14100 21300 13000 30 26 4700 31700 32200 15300 31 33 6500 20500 21400 11100 32 73 10400 12500 18100 —

Alloy 32 does not satisfy the conditions 4×(Cr+Mo)≧125−3×Ni, and its magnetic permeability is very inadequate. 

1. Magnetic shielding for frequency fields between 50 Hz and 3000 Hz, made from a soft magnetic alloy of the following composition in % by weight: 30%≦Ni≦40% 0%≦Cu+Co≦4% 5%≦Cr+Mo≦17% 5%≦Cr 0%≦Nb≦2% Mn≦0.35% Si≦0.2% C≦0.050% O≦0.0160% S≦0.0020% B≦0.0010% optionally at least one element selected from magnesium and calcium in amounts such that the sum thereof remains below 0.1%, the remainder being iron and production impurities, the chemical composition furthermore satisfying the following relationship: Cr+Mo≦0.8×Ni+0.9×(Co+Cu)−18.4 Cr+Mo≦4×Ni+3×(Co+Cu)−124 4×(Cr+Mo)≧125−3×Ni.
 2. Shielding according to claim 1, further characterised in that: Si≦0.15%.
 3. Shielding according to claim 1, further characterised in that: Mn≧0.05%.
 4. Shielding according to claim 1, further characterised in that: Co+Cu≧0.015%.
 5. Shielding according to claim 1, further characterised in that: O≧0.0050%.
 6. Use of a soft magnetic alloy of which the composition is as defined in claim 1, for the production of magnetic shielding for frequency fields between 50 Hz and 3000 Hz. 